This invention pertains to excimer lasers and more particularly to the conversation of the gases used in an excimer laser.
One of the most successful new laser technologies to have emerged in recent years is a new family of powerful gas lasers called excimer lasers. These lasers are unique in that they are efficient sources of high powered ultra-violet light. This property makes these lasers very useful for a wide range of scientific as well as industrial and military operations.
An economic limitation to the long-term use of laser of excimer lasers had been the high cost of operation. A significant portion of this cost has been due to the consumption of expensive rare gases such as krypton, argon or xenon as well as moderately expensive gases such as helium and neon. Ordinarily, the lasing gas mixture which is a combination of a heavy rare gas, a diluent gas and a halogen gas is fed into the laser and withdrawn from the laser after the lasing has been performed. Because of the cost of the rare gases, the excimer laser can be expensive to operate. However, the present inventor was part of a team which developed a closed-cycle recirculating system for rare gas halide excimer lasers. The results of this invention were published in Applied Physics Letters, Volume 32 (5) on Mar. 1, 1978, starting on page 291 thereof. (This idea is also discussed in Excimer Laser Chemical Problems, Los Alamos National Laboratory document Number Q-8-L-169..) While the proposed system has performed admirably to reduce the cost of the gases by virtue of the fact that the heavy rare gases are cleaned and recirculated after the removal of the halogen and fed back to the laser along with fresh halogen gas, there is a safety problem with such a system when pure halogen gas is used. This is because Halogens are poisonous gases in concentrations greater than 0.1-1 ppm. Furthermore, Halogens are very corrosive. In fact, pure flourine gas is very pyrophoric, and the greater the concentration, the greater the chance for cuasing a fire. If the source of the halogen is hydrogen chloride, then there are similar problems. Therefore, excimer laser users tend to use the halogen gas in a dilute form with a diluent gas such as helium. The gas mixtures of halogens and diluent gases are available from sources who have the equipment and knowhow to safely handle the halogens so the laser users do not have to use pure halogens. Nevertheless, in a closed cleanup and recirculation system the feeding of this dilute halogen gas mixture causes a raising of the pressure of the system, since the diluent gas is not removed from the gas stream by the cleanup process. This then requires that the system be vented in order to maintain constant laser pressure. In the course of venting, the proportions of the rare gases will change, as the halogen diluent flushes out the initial gas mixture. This will cause laser performance to deteriorate. The alternative to venting the gas mixture is to use a helium diffuser to selectively remove excess helium. For the gas flow rates encountered in excimer laser cleanup systems, a helium diffuser would have to provide very high rates of diffusion such that the diffusion system would be large, expensive, and probably very fragile, adding little to the overall safety of the system.